A basic capacitor is made up of two conductors on which equal but opposite electric charges are placed, and an insulator, which is also called a dielectric, separates the two conductors. This dielectric could be made of paper, plastic, mica, ceramic, glass, or almost any other nonconductive material. Because each conductor stores an equal but opposite charge, the total charge in the device is always zero. The electron storing ability of a capacitor is called "capacitance" (C), and it is measured in Farads. The capacitance (C) is a measure of the amount of charge (Q) stored on each conductor (plate) for a given potential difference or voltage (V). The formula that represents this relation is C = Q/V. In SI units, a capacitor's capacitance is one Farad, which means one coulomb per volt.
Since the Farad is a very large unit, capacitors are usually rated in microfarads (mF=106F), nanofarads (nF=10-9F), and picofarads (pF=10-12F).
Something else that is involved with capacitors is the stored energy. When a capacitor is being charged, voltage is developed across the capacitor as a result of the electric field created by the accumulated charge. The energy, which is
Cited: Dummer, G. W. A. Fixed Capacitors: Radio and Electronic Components. Vol. 3. 1996. Van Roon, Tony "What Exactly Is A Capacitor?" 2001 http://www.uoguelph.ca/~antoon/gadgets/caps/caps.html